Washington, Aug. 20 (Reuters): A new oral impotence treatment called Levitra has won US approval, providing the first challenge to the popular Viagra in the world’s most profitable market for medicines.

Levitra, an orange pill made by GlaxoSmithKline Plc and Bayer AG, will give men an alternative to Pfizer Inc.’s diamond-shaped blue pill, the drug that transformed impotence treatment after its 1998 debut.

Viagra gained instant success as the first oral therapy for impotence, but Levitra’s makers are planning a major marketing blitz to lure many of the millions of men who have not sought treatment. An estimated 30 million US men experience some level of erectile dysfunction.

“We know, from considerable market research, that the market is ready for new options,” said Lawson Macartney, head of strategic management of GlaxoSmithKline’s cardiovascular, metabolic and urology drugs.

Glaxo expects Levitra to be available in September. Another competitor, a pill called Cialis, also may hit the US market later this year. All three drugs already are competing in Europe.

Levitra, Viagra and Cialis all work by blocking an enzyme called PDE-5, which affects blood flow to the penis, but the potency, speed and duration of each drug are hotly disputed.

Levitra’s makers say a major plus is that it works quickly. In clinical trials, some men achieved an erection in 16 minutes.

Pfizer has responded with studies showing half of men taking Viagra were able to have sex within 20 minutes.

Trials of Cialis, from Eli Lilly and ICOS Corp, shows it gives patients a window of up to 36 hours in which to have sex. The window is about four hours for most men taking Viagra and up to five hours with Levitra.

Levitra’s most common side effects include headache, flushing and nasal congestion. Dizziness was reported in about 2 per cent of patients. Headaches, flushing and stomach up- set are Viagra’s most common side effects.

The Food and Drug Administration, in a statement announcing Levitra’s approval, said Levitra should not be taken with either of two types of drugs — nitrates and alpha blockers — because the combination may lower blood pressure and lead to fainting.

Macartney said pricing of Levitra will be competitive with Viagra. In Britain, Levitra costs the same as Viagra.

Pfizer said it believes it can handle the competition and plans no changes to its marketing of Viagra, which had 2002 sales of $1.74 billion.